I am not sure exactly what the overstay fees are right now, and really it changes depending on who you are dealing with at Immigration.
But yea, I did the math some years ago and saw that it was much cheaper for me to stay and pay the fine, then to go out every 90 days.
The first time I paid the fine was for 3 years overstay, and it cost me about 21,000Lps. So yea, that 7500L per year looks about right.
But that was before COVID...